This invention relates to the field of switching type AC to DC power supply converters and, more particularly, is directed to a method and apparatus for converting an AC voltage and current to a highly stable constant DC voltage such that the current drawn from the power source has a predetermined waveform.
Switching type AC to DC converters are widely used to provide regulated power supply voltages. The voltage bucking type converter represents one example of such a converter. In this type of converter, the input voltage is "bucked" to produce a lower value of output voltage. A simplified example of such a converter is shown in FIG. 1.
The buck converter of FIG. 1 includes switching transistor 1 which receives a positive source of voltage at its collector electrode. The emitter electrode of transistor 1 is connected to choke coil 2 and the cathode end of diode 3. Capacitor 4 serves as a voltage filter for the output voltage at nodes B-C which is supplied to load 5.
When transistor 1 is initially turned on by a control voltage applied to its base electrode, full source voltage appears across choke coil 2 because the initial current flow through the coil is zero. Current flow begins to increase over time and the output voltage at nodes B-C begins to rise. When the output voltage reaches a value slightly above the desired output level, transistor 1 is turned off. The voltage across nodes A-D becomes negative as the magnetic field around choke coil 2 collapses. Thus, diode 3 conducts and the energy which has been stored in coil 2 is dissipated into load 5 through diode 3. As this energy is dissipated, the current flow through coil 2 and the output voltage at nodes B-C begins to decrease. When the output voltage drops to a level slightly below the desired output level, transistor 1 is turned on again and the output voltage begins to rise to begin another cycle.
In order to provide proper and efficient operation of a buck converter, the switching device, i.e., transistor 1 in FIG. 1, must be precisely controlled. Such a control device is set forth in commonly assigned co-pending Application Ser. No. 123,720 filed Nov. 23, 1987 entitled "AC to DC Power Converter With Integrated Current Control", now U.S. Pat. No. 4,816,982, issued Mar. 28, 1989, which is incorporated herein by reference.
The circuit elements of FIG. 1 may also be configurated as a "boost" converter to provide an output voltage which is higher than the source voltage. The operation of a boost converter will be discussed below in connection with Applicants' invention.
The fundamental challenge of power supply design is to simultaneously realize two conflicting objectives: maximizing the electrical delivery performance of the power supply while at the same time achieving a power supply design of low cost. To this end, control circuitry for switching type AC-DC power converters has evolved which includes a pulse width modulation circuit operated at a frequency much higher than that of the alternating current input. The pulse width modulation circuit in turn activates the converter switching device for switching the rectified line current in accordance with an applied pulse width modulated signal. The pulse width modulation circuit is actuated by the result of a comparison of the input voltage waveform and an error signal obtained by subtracting a reference voltage from the voltage delivered to a load of the power supply.
According to the teachings of Pacholok, U.S. Pat. No. 4,472,672, an improved power factor results from forcing the output impedance of such a circuit to appear to be substantially purely resistive. Consequently, while Pacholok is capable of maximizing electrical delivery to a load, he does so at the expense of costly components such as a stepdown center tap transformer for input voltage waveform sensing which make the converter uneconomical for practical application.
Retotar, U.S. Pat. No. 4,591,963, discloses a similar technique to that employed by Pacholok in the sense that the input voltage waveform is sensed and a pulse width modulator is controlled responsive to a means for combining the sensed input voltage waveform and the influences of the output voltage waveform delivered to a load upon a reference voltage. What Retotar adds to the technique disclosed by Pacholok is the application of line current sensing techniques for the purpose of constraining current in an input inductor to be in phase with the input voltage waveform. While line current sensing techniques are appropriate for control of peak current, any improvement in electrical power delivery to a load requires the application of substantially the same input line voltage waveform sensing technique as taught by Pacholok.
One problem with input line voltage waveform sensing techniques has been the inability of the control circuitry to react quickly to transients appearing in the line voltage By the time an input voltage transient is reflected in the load voltage waveform, it is too late for control circuitry to efficiently react.
Nevertheless, the control circuitry itself has evolved to such a state that economical special purpose integrated circuits have been introduced into the marketplace for application with such voltage control techniques. These low cost integrated circuits facilitate the maximization of electrical power delivery at low cost. For example, the Unitrode Corporation UC 3842 integrated circuit includes in a single chip an oscillator, an error amplifier, a PWM latch or flip-flop and current sensing and limiting circuits at its input. At its output, a control signal may be provided for controlling, for example, the switching transistor of a current mode controlled buck regulator. From the perspective of all such known applications, there still remains a requirement to overcome the fundamental challenge presented above of achieving excellent electrical performance at low cost in an AC to DC converter and to improve transient response.
In addition, the ideal AC to DC converter should provide a highly stable output voltage such that the current drawn from the power source has a predetermined waveform. In a commercial AC system, the most desirable output current waveform is a sine wave which is in phase with the voltage waveform in order to provide a unity power factor. Unity power factor offers the most efficient utilization of power line generating capacity. AC to DC converters known in the prior art are not satisfactory in this regard. One such attempt to provide unity power factor in an AC to DC converter is disclosed in an article by Richard Keller entitled "Unity Power Factor Off Line Switching Power Supplies," IEEE publication, pg. 332-339 (1984). In converters of the type disclosed in the Keller article, however, the regulated DC output voltage is always larger than the maximum input peak voltage in the power line. For example, where the input power line voltage is 120 volts, the output voltage will not be less than 200 volts. Such a limitation creates a significant problem when the desired output voltage must be less, e.g., 24, 48 or 100 volts DC. In such cases, an additional DC to DC converter is required in order to reduce the voltage, thereby increasing the expense, size and weight of the power supply.
In addition, the current waveform drawn from the power line is exactly the same as the voltage waveform in the power line. Prior art AC to DC converters do not have the flexibility of producing different waveforms and thus cannot achieve a high power factor without sacrificing other requirements. As pointed out above, another shortcoming of prior art converters is that transient response is poor. A change in input voltage causes a corresponding change in current and output voltage before feedback from the output brings the system back into balance. Such a slow transient response is unacceptable in power supplies which require a highly regulated output.